TriPolar for the win!
The US exports vastly more “culture” than it imports. Thus, non-US folks know about the US much more than US folks know about the non-US. Take someone in an economic and social setting in the US that has little contact with anything British. If you take someone in the UK who is similarly positioned economically and socially, they will have contact with many American things, even if it’s just from watching a Hollywood movie once in a while.
It isn’t just the lack of exposure to British, etc., culture. Not only do you have to watch British movies or TV shows, but they have to clearly indicate in the plot that they are pronouncing the last letter of the alphabet, and that pronunciation is “zed”, not “zee”. That doesn’t happen too often. A casual reference is likely to go right over the heads of most people.
Well, I know, but who cares? I mean the Brits are slowly changing over, and they understand what we mean by “Zee” (just like I understand what they mean by “Zed”), so is it really a big deal? I also understand what they mean by “fag” “lorrie”, etc also.
Is this where I get to make fun of my German relatives who visited and seriously thought we could see the Empire State Building, Grand Canyon, Disneyland, Cowboys and Indians, and Willie Nelson on an 8 day road trip from the Washington state?
Or do we move on to being shocked at who doesn’t know how correctly pronounce aluminum and lieutenant?
I think the OP significantly overestimates
A) How available non-U.S. popular culture is in the U.S. without seeking it out compared to how available U.S. popular culture is in other countries without seeking it out.
B) How common, when consuming such things, it is for someone to say “Z” out loud.
C) When “Z” is said out loud it is in a situation important enough for the listener to go “zed?, what’s zed? That’ makes no sense. Really? Zed is how Brits pronounce Z? Interesting, I’ll remember that.”
Similarly, I watch 20 Italian movies a year and couldn’t tell you what an “Exit” sign looks like in Italy even though I’ve probably seen hundreds of them in movies. It just isn’t important enough to embed itself in my brain.
I do know “zed” but I honestly couldn’t tell you how I came to know that or when.
“Lorrie” 
… Oh, you mean lorry!
I am sure that many, even most, Brits do not know that Americans call the last letter of the alphabet zee (although perhaps more Brits, proportionately, know than Americans know about zed).
My suspicion is that more people know than care.
I had a book when I was a little kid that went through the alphabet A to Zed. My grandma brought it home on one of her trips.
That said, is the book and upcoming Brad Pitt movie pronounced “World War Zed” over there?
I know Zed is “Z” and I still don’t get it. I thought Zed was just the thing they were calling the guy. It makes more sense if it indicated the letter?
And pronouncing the “h” in herb. And I don’t think any of those things are extra; they’re just different.
I suspect the Canadian “zed” may eventually go away. I myself probably say “zee” more than “zed,” as do a lot of people my age. If you asked my kid or any of her schoolmates to sing the alphabet song, they’ll probably finish with “zee.”
Part of this is American influence, but part is just that “zee” makes more sense. And sounds better in the song.
I know Z=Zed, have for a long time but I watch movies with subtitles, have a passport and tour with a Mancusian and a guy from Bristol( whatever they are called). One of our favorite(favourite) pastimes on the road is the differences between our uses of our shared language.
One thing I have trouble with is the er/re ending. How in the heck can they be pronounced the same way? That is an E on the end not and R. Yet, somehow, we still get along.
I have a Glaswegan buddy, I have to translate for him after a few beers because no one else can understand him.
Oh and Orcs come from Tazmania, Elves and Hobbits come from New Zealand 
Capt
PS we have to work out the date thing, please. I am always confused whenever it comes up. Either way is fine by me.
Mancunian.
Bristolian.
Glaswegian.
Tasmania. And, believe it or not, Tasmanians can also be referred to as “Taswegians” colloquially.
Thank you. I can talk but my spelling leaves something to be desired and none of those words will spellcheck for me.
Capt
ETA He says it as Mancusian not Mancunian FWIW
Cheers
Maybe I’m wrong about that one and there is another demonym I’m not aware of, assuming you mean Manchester, England. I am not from the UK, but “Mancunian” is the more usual name for residents of Manchester. But language is living and all that, so perhaps there is a shift to “Mancusian” these days. I’m actually curious about this, so if anyone can chime in, please do.
Yes Manchester UK, his dad is the one who really uses Mancusian and he is in his sixties. My tour mate uses this around his dad as he was born there but not raised there. Please someone who knows chime in. Dad has lived in the US for 25 or so years.
Capt
I’m British, and I’ve only ever heard Mancunian.
A search on the Manchester Evening News website has thousands of “mancunians” and no “mancusians”
Well Dad is insane, not much of a surprise to me. Thanks all and ignorance fought.
Now can we settle the date thing?
Capt
Everyone should use ccyymmdd.
From Manchester = Mancunian or Manc. I can’t actually think of any other Manchester-specific demonyms.
Aluminium.